The favorable nuclear properties of 57Fe facilitate the study of heme iron by Mossbauer Spectroscopy. This iron-specific probe senses electrostatic and magnetic features of the local electron distribution, providing in many cases information not available through other physical measurements. Using and extending existing methods of measurements and analysis, this technique will be applied in an effort to understand the nature and mode of action of the active centers in hemoproteins in terms of the active site conformation and the relevant electronic wave functions. We intend to exploit promising possibilities opened up by our low temperature photodissociation work and by our preliminary success in the interpretation of Mossbauer lineshapes in terms of conformational excitation of bound oxygen. Hopefully this will advance our understanding of the reversible iron-oxygen bond, the effect of steric hindrance to the oxygen, and the modification of the binding site in hemoglobin cooperative interaction. We believe the iron-oxygen bond studies will be facilitated by extension of our observations to abnormal and artifical myoglobins (e.g., deuteromyoglobin), and by continued study of model compounds. Study of the unliganded iron site in normal myoglobin via its magnetic Mossbauer hyperfine interactions is well under way and will continue. Studies bearing on cooperativity will involve investigation of abnormal and modified hemoglobins, in order to see which aspects of our photodissociation results are related to cooperative effects. Both protein and model compound work will be carried out in collaboration with established workers in these fields. The main experimental technique will be Mossbauer spectroscopy, and we are now fairly well equipped for the Mossbauer measurements over an adequate range of temperature and magnetic field. The computational techniques for data reduction and interpretation are well in hand, and we are progressing with theoretical efforts to treat ever more complicated problems in terms of electronic wave functions in an effective crystal field. Because the Mossbauer spectra and susceptibilities are related and complementary, we wish to extend our capability to susceptibility measurements. It is our intention to make a preliminary study which could lead to the construction of a somewhat novel Faraday susceptibility balance.